VISAKHA BUCHA (Vesak) means the worship of the Buddha on the full moon day of the sixth lunar month. It usually falls in May. In the case of a year with an extra eighth lunar month--Adhikamasa (there are 13 full moons in that year)-- the Visakha Bucha Day falls on the full moon day of the seventh lunar month.
The Vesak full moon day (The full moon day of Sixth lunar month)
Visakha Bucha Day is one of the most important days in Buddhism because of three important incidents in the life of The Buddha, i.e. the birth, the enlightenment and the passing away, miraculously fall on the same month and date, the Vesak full moon day. .So each year, Buddhists throughout the world gather together to perform the worship to recollect the wisdom, purity and compassion of the Buddha.
The performance of the rituals on Visakha Bucha day had been continuously observed in Jambudavipa or India, the motherland of Buddhism, for a long time before Buddhism spread to Sri Lanka and Thailand. There, the Visakha worship has been continually observed to the present day.
In Thailand, Visakha Bucha observance began during the Sukhuthai period (around 700 years ago), because of the close religious relations between Thailand and Sri Lanka. Sri Lankan monks came to propagate Buddhism in Thailand and were highly respected. Thai monks also went to study in Sri Lanka. It's believed that, those monks introduced this ceremony to the King and people at that time